Known from WO 2006/108451 A1, for example, is a speed-regulated force-limiting device for a seat belt retractor which features several masses, which are positioned on the frame of the seat belt retractor and are to be put in oscillatory motion. The masses are mounted on the retractor frame so as to rotate and alternatively engage with two teeth during the force-limited seat belt extraction into a gear ring connected to the belt shaft. Overall the seat belt retractor is complicated and expensive to manufacture because of the oscillating masses positioned on the retractor frame. Furthermore, because of the oscillating masses, the seat belt retractor requires a relatively large installation space.
An improved force-limiting device operating on the basis of the same physical principle is described in German Patent Application DE 10 2008 049 931.5, in which two toothed parts are moved in a force-limited manner toward one another and one of the parts thereby executes an undulating feed motion during which the toothed parts alternatingly engage and disengage. The oscillating masses are replaced in this embodiment by the part executing the undulating feed movement, so that the force-limiting device described there requires an essentially smaller installation space and has a substantially simpler design. In one embodiment, the part executing the undulating feed movement is in the form of a toothed disc which can be connected in a friction-locking manner to the belt shaft. Between the toothed disc and the part against which the toothed disc executes the feed movement, a spring washer can be provided which exerts an axial compression force on the toothed disc, whereupon the force-limiting level of the force-limiting characteristic is increased.
The force-limiting level of the force-limiting device automatically results from the kinetic energy of the passengers during forward displacement, which must be reduced, wherein the basic characteristic of the force-limiting path is strictly determined by the parameters of the toothed disc, such as the number of teeth, the separation of the teeth, the mass of the toothed disc and by the spring characteristics of the spring washer.
There are legal requirements, however, such as US requirements for restraining forces for passengers in the back seats, which mandate an increase of the restraining forces to a higher force-limiting level after a preset seat belt extraction. Such a force-limiting characteristic is not obtainable with one of the above force-limiting devices.
The object of the invention is to create a force-limiting device of this type which enables a progressive force-limiting course of the restraining forces and can be constructed simply.